As is known, the function of a wind power turbine is to convert the kinetic energy of wind into electrical energy. The electric current produced regeneratively in this way can be fed into a local or inter-regional power grid for the supply of electricity to consumers.
For the more powerful wind power turbines, a design has become established, in which a wind rotor, preferably comprising three rotor blades, is arranged on the windward side of an enclosure, to rotate about a horizontal rotational axis. The enclosure is mounted, by means of an azimuth bearing, to rotate about a vertical axis on a tower, which is fixed firmly in the ground by a foundation. The enclosure accommodates at least one step-up gearset and at least one generator for producing electrical energy. By virtue of the step-up gearset, whose input shaft is connected to the rotor hub of the wind rotor, the relatively low speed of the wind rotor is converted to a higher speed appropriate for the generator, with a corresponding change of the torque transmitted to the output shaft in direct or indirect driving connection with the rotor of the generator.
Older wind power turbines are often of fixed-speed design, i.e. in these wind power turbines, the speed of the wind rotor and hence also the rotor speed of the generator are kept constant by adjusting the angle of incidence of the rotor blades (pitch regulation) and/or by turning the rotor in its azimuth bearing away from or into the wind (stall regulation). In this way, with appropriate design of the transmission and the generator, the current produced by the generator, preferably designed as a synchronously running unit, can be fed into the power grid and this, it is true, without much power electronics cost and complexity, in particular without an expensive frequency inverter. However, the disadvantage of fixed-speed wind power units is the relatively small range of wind speeds in which they can operate effectively.
To expand the useful wind speed range and thereby increase the energy yield, wind power turbines have been developed in which both the wind rotor and the generator can operate at variable speeds. However, a variable-speed generator requires a frequency inverter by which the voltage, frequency and phase position of the electric current produced can be adapted so as to be compatible with the power grid conditions. Since all the electric current produced in the generator passes through the frequency inverter the latter has to be of correspondingly high-power design, but because of the increasing power of modern wind power plants this is associated with considerable costs and high breakdown potential.
Accordingly, to avoid these disadvantages wind power turbines have also been proposed, which comprise a wind rotor that can operate at variable speeds, a generator that operates at constant speed and a force regulation device, which are in driving connection with one another by way of a superimposition transmission. The superimposition transmission is preferably designed as a simple planetary transmission whose planetary carrier (web) is connected directly or via a step-up gearset to the hub of the wind rotor, whose sun gear is connected to the rotor of the generator or the force regulation device, and whose ring gear is connected to the force regulation device or the rotor of the generator. By fixing the ring gear or sun gear in forward rotation (same rotational direction as the wind rotor and generator rotor) or backward rotation (rotational direction opposite to that of the wind rotor and generator rotor) by means of the force regulation device, the gear ratio of the planetary transmission acting between the planetary carrier and the sun gear or ring gear can be regulated as a function of the variable speed of the wind rotor in such manner that the rotor speed of the generator is kept substantially constant. The rotor speed of the generator can additionally be influenced by pitch regulation and/or stall regulation.
In a drive-train that can be used in a wind power turbine according to DE 103 14 757 B3, the force regulation device is preferably designed as a hydrodynamic torque converter arranged coaxially over the output shaft that connects the sun gear of the superimposition transmission to the rotor of the generator. The pump impeller of the torque converter is connected in a rotationally fixed manner to the output shaft, whereas the turbine wheel of the torque converter is in driving connection with the ring gear of the superimposition transmission by way of a step-down gearset designed as a simple planetary transmission with a fixed planetary carrier (static transmission with rotation direction reversal), which is in driving connection with the ring gear of the superimposition transmission. To control the speed and torque absorbed by the pump impeller, the vanes of the torque converter can be adjusted.
As explained in more detail in the associated DE 103 61 443 B4, in such a drive-train, with an appropriate design of the superimposition transmission, the torque converter and the step-down gearset, and with an appropriate setting of the deflector vanes of the torque converter, passive—i.e. largely automatic—regulation can be achieved, by virtue of which, within a predetermined wind speed range, the wind rotor can be operated with variable speed at its optimum operating point in each case and at the same time the rotor speed of the generator can be kept constant. However, the disadvantage of this known wind power turbine is that despite the fact that the wind rotor operates at optimum efficiency, the overall efficiency is comparatively low because of the continual back-flow of energy with poor efficiency through the torque converter.
In contrast, DE 37 14 858 A1 describes a transmission that can be used in a wind power turbine, in which the power control device is in the form of a second generator with lower power compared with the first generator (the main generator). In the embodiment variant shown in FIG. 3 of that document the rotor of the main generator is connected in a rotationally fixed manner to the ring gear of a superimposition transmission, whereas the rotor of the second generator is connected in a rotationally fixed manner by way of a driveshaft to the sun gear of the superimposition transmission. The second generator is positioned coaxially, axially behind the main generator as viewed from the wind rotor, so that the associated driveshaft runs centrally through the hollow rotor of the main generator. By appropriate regulation of the rotor speed and the torque absorbed by the second generator as a function of the rotor speed of the wind rotor, the rotor speed of the main rotor in high wind speeds can be kept constant. In this case the second generator produces electric current additionally to the main generator, although due to the variable-speed operation of that generator it has to be modulated by an associated frequency changer before being fed into a power grid.
From the description of a wind power turbine which is structurally slightly different but functionally identical in EP 1 283 359 A1, it is known that an electric machine provided as a power control device for keeping the generator's rotor speed constant can be operated both as a generator and as a motor. When the electric machine is operated as a generator, i.e. with the rotor of the electric machine and the sun gear of the superimposition transmission rotating in the same direction as the wind rotor and the rotor of the generator, the gear ratio acting between the wind rotor and the output shaft of the step-up gearset and the rotor of the generator becomes higher as the speed of the wind rotor increases compared with the condition when the sun gear is immobilized, which corresponds to operation of the wind power turbine at high wind speeds above the rated rotational speed of the wind rotor. During motor operation of the electric machine, i.e. when the rotor of the electric machine and the sun gear of the superimposition transmission are rotating in the opposite direction to the wind rotor and the rotor of the generator, the gear ratio acting between the wind rotor and the output shaft of the step-up gearset and the rotor of the generator becomes lower as the speed of the wind rotor decreases compared with the condition when the sun gear is immobilized, which corresponds to operation of the wind power turbine at average wind speeds below the rated speed of the wind rotor. During generator operation of the electric machine the additional electrical energy produced is fed into the power grid by way of an associated frequency inverter, whereas when the electric machine is operated as a motor the electrical energy needed for this is drawn from the power grid via the frequency inverter.
Generally, however, to further increase the energy yield of wind turbines it is desirable also to extend their operating range down to lower wind speeds.